X-ray apparatus



March 7, 1944. z. J. ATLEE 12,343,729

' X-RAY APPARATUS Filedfluly 10, 1942 INVENTORI- Z667 JalZ f,

Patented Mar. 7, 1944 X-RAY APPARATUS Zed J. Atlee, Elmhurst, Ill., assignor to General Electric X-Ray Corporation, Chicago, 111., a corporation of New York Application July 10, 1942, Serial No. 450,419

10 Claims.

The present invention relates in general to electronics and has more particular reference to means for and method of operating an X-ray generator in order to preserve the anode target thereof against the destructive forces to which the same is subjected when the generator is in operation with the target under electronic bombardment.

X-ray generators, which in general comprise an anode and an electron emitting cathode enclosed in a sealed, evacuated envelope, operate to produce X-rays at the anode by the impingement thereon of electrons emitted by the cathode and caused to impinge upon the anode under the electron driving infiuence of electrical potential applied between the anode and cathode.

During operation, the anode becomes excessively heated, particularly at the surface thereof which receives the impact of the electrons, such surface comprising the electron target of the anode and being ordinarily formed as a button of heat resisting material, such as tungsten, supported on and in intimate thermal relationship with the body of the anode which commonly comprises heat absorbing the dissipating material, such as copper or a copper alloy, the anode base serving to Withdraw heat from the target as rapidly as possible in order to preserve and protect the same against the destructive effects of excessive heating.

X-ray generators, especially when used in the making of X-ray pictures, are commonly operated during exceedingly short exposure intervals, ranging from intervals of the order of one-twentieth of a second and sometimes lower, up to intervals of the order of two seconds or more; and it has been observed that fractional second loadings, particularly those of less than one-tenth of a second in duration, are exceedingly detrimental to serve life of high current radiographic X-ray generators, service life of generators being foreshortened due to anode failure through cracking of the target service.

The foreshortening of life is most apparent where the generator is used for general radiographic purposes where the exposure intervals may vary from one-twentieth of a second to several seconds. A generator which is never used in making exposures of a duration of less than onefifth of a second ordinarily does not develop cracks and consequently has a service life of indefinite length so far as anode failure is concerned. Cracking occurs usually when the generator is employed during exposure intervals of the order of one-tenth of a second or less; after the initiation of such a crack, exposures of onefifth of a second or longer disrupt the target rapidly and failure usually then takes place after one thousand to three thousand exposures. On the other hand, if only fractional second exposures of less than one-tenth of a second are used, the generator will ordinarily have a reasonably long service life, even though the target becomes cracked shortly after the generator is placed in service.

An important object of the present invention is to prevent cracking of the anode target; and thereby to indefinitely extend the service life of X-ray generators, regardless of the exposure intervals required of the device; a further object being to provide a method or technique for the operation of generators whereby to avoid anode cracking in service.

Another important object resides in so controlling a generator as to prevent the development of anode cracks through excessive heating and rapid cooling thereof when the same is in operation during fractional second exposure periods; a further object being to provide means for assuring the development of sufiicient heat in the anode during fractional second loadings to prevent the disruptive efiect on the anode surface of excessively rapid cooling thereof following the fractional second loading.

These and numerous other important objects, advantages, and inherent functions of the invention will be fully understood from the following description, which, taken in connection with the accompanying drawing, discloses a preferred embodiment of the invention.

Referring to the drawing:

The figure is a diagrammatic showing of an X-ray generator and means for operating the same in accordance with the teachings of my present invention. 1

To illustrate the invention I have shown on the drawing an X-ray generator ll comprising an anode [3, a cathode l5 and a sealed envelope l'i enclosing the anode and cathode. Any suitable means for energizing the cathode for electron emission may, of course, be employed.

Likewise any suitable system for applying potential between the cathode and anode for impelling electrons to the anode may be used, although for purposes of illustration, the drawing indicates a network of rectifiers I9 connected to the anode and cathode of for delivering operating potential therebetween from a transformer 2|, the secondary Winding 23 of which is connected to the rectifying netthe X-ray generator work. The primary winding 25 of the transformer is connected by conductors 2! through switch means 29 and an auto transformer 3| with a suitable source of electrical power 33 so that generator operating potential may be delivered between the anode and cathode of the generator when the switch means 29 is closed.

Operation of the switch means 29 is controlled by a solenoid or relay 35, the armature 31 of which is drivingly connected with the switch means 29, the solenoid 35 being connected in circuit with a timer switch 39 and a control switch 4| whereby the solenoid, under the control of the switches 39 and 4|, may be energized from the power source 33. The timer switch 39 preferably comprises a device of the sort illustrated and described in the copending application of Arthur J. Kizaur, Serial No. 400,666, filed July 1, 1941, upon which Letters Patent No. 2,325,860 issued August 3, 1943, and serves to maintain the solenoid circuit only during an accurately predetermined time interval following the closure of the switch 4|.

In order to operate the generator H for the production of effective X-rays during a desired exposure interval, the cathode l5 being properly energized to a desired degree for the production of electrons at the cathode, and the switch 4| being open, the timer switch 39 is set for the desired exposure interval and the switch 4| is then closed to energize the solenoid 35, which in turn closes the switch means 29, thereby applying generator actuating potential between the anode and cathode of the generator. At the same time the timer switch 39 is set in operation to open the solenoid circuit after the expiration of the time interval for which the switch 39 has been set, the solenoid 35 being de-energized by the timer switch upon the expiration of such set interval, thereby opening the switch means 29 and discontinuing the application of generator operating voltage between the anode and cathode of the generator.

Upon the application of generator operating voltage between the anode and cathode of the generator, intense electron bombardment of the target surface of the anode is initiated, thus releasing, as heat, large amounts of electricalenergy in the anode. The electron bombardment of the anode results initially in exceedingly rapid heating of the surface portions of the anode target. The large mass of the anode structure, however, becomes heated much more slowly than the surface portions thereof. The major portion of the anode, of course, including the heat dissipating body thereof, becomes heated much more slowly than do the surface portions of the target. This large mass of heat absorbing and heat dissipating material in the body of the anode is, however, adapted to absorb heat rapidly so that if anode loading be continued for an appreciable interval, say of the order of one second or more, a substantial quantity of heat will become stored in the body of the anode behind the target surface. If, however, application of generator operating potential be discontinued after a relatively short period of time, of the order of one-tenth of a second or less, and before the body of the anode has received much heat, the anode body, at relatively low temperature, will cause exceedingly rapid cooling of the target surface from a relatively high temperature, and it is thought that such excessively rapid cooling is responsible for the development of anode cracks. The phenomena has been investigated as described in an article entitled Transient temperatures in the anode of an X-ray tube, by F. R. Abbott, in Journal of Applied Physics, volume 13, No. 6, June, 1942, pages 384-389.

Where generator operating potential is maintained for a period of time in excess of one-tenth of a second, the temperature of the body of the anode becomes appreciably higher than the temperature reached by it after a shorter interval, the temperature of the anode body after a halfsecond interval being several hundred degrees centigrade higher than the anode body temperature reached after an interval of onetwentieth of a second. After an interval of one second, the body temperature of the anode may be at a temperature in excess of 1000 degrees centigrade, whereas after an interval of onetwentieth of a second it may only be at a temperature of the order of a few hundred degrees centigrade.

Reasoning from the foregoing data, which has been discovered as a result of careful investigation, it is concluded that the high incidence of anode cracking in generators when subjected to fractional second loadings is due to the relatively small amount of heat stored in the body of the anode and the consequent relatively low temperature of the anode body as compared with the temperature of the target surface at the conclusion of a fractional second loading; and that the reason why cracking does not occur to any appreciable extent where generators are loaded during exposure intervals of the order of one-fifth of a second and higher is because, after such extended loadings, the temperature of the body of the anode has been raised to a point where the target surface may cool sufficiently slowly to prevent cracking because of the retarded cooling effect afforded by the storage of appreciable quantities of heat in the anode body immediately behind the target surface.

In order to make use of the foregoing heating and cooling characteristics for the prevention of anode cracking, I provide means for maintaining reduced potential between the anode and cathode of the device following short exposure intervals. By thus maintaining a reduced potential on the generator, I continue to raise, or at least to maintain, the temperature of the body of the anode by delivering electrical energy thereto through the target surface thereof, and, at the same time, modify the cooling of the target surface and prevent excessively rapid cooling of the same.

The reduced potential thus maintained between the anode and cathode of the generator following short exposure intervals need be only a fraction of the potential required for the operation of the device and may be of such small dimension as to result in the production of no effective X-rays, it being understood that, for each generator there is a minimum operating potential value below which the generator will not function to produce effective X-rays and the present invention provides means for continuing the application of potential between the anode and cathode of the generator following a fractional second operation of the device for the production of useful X-rays at a value such that effective X-rays are not produced, while at the same time regulating the cooling of the anode in the manner described whereby to prevent the development of anode cracks.

Another method by which the anode may be kept at a sufiicient temperature to prevent crackcooling within safe limits, since extending the pe-.-. riod longer than necessary to prevent cracking ing is to provide an extra anode heating element, such as a Calrad unit, in the housing, or, by suitable insulation transformer, in the anode itself by providing a hollow anode shank. The use of an additional heating unit, however, is objectionable because of expense and because, in order to maintain the anode at all times at a sufficiently high temperature to prevent cracking, the extra heater so increases the total heatsupplied to the generator as to seriously reduce the duty cycle ratings of the tube and housing. By the method of maintaining a reduced potential between anode and cathode for a limited time interval after short exposures, only a minimum of heat is added, so that the duty cycle ratings are not materially affected.

In order to maintain a reduced potential between the anodeand cathode of the generator I I following short exposure intervals, I merely provide auxiliary switch means 43 connected between the secondary winding 25 of the transformer 2| and the power source 33 through the auto transformer 3 I, the auxiliary switch 43 being in parallel relationship with respect to the switch means 29. The auxiliary switch means 43 is connected in circuit with a resistance unit 45 or other potential reducing means sufficient to drop potential supplied through the auxiliary switch means to the generator to a value below the voltage value required to produce useful X-rays at the generator. The switch means 43 is operatively connected with the armature 46 of a relay or solenoid 41 which is connected in an operating circuit including a control switch 49, said operating circuit being connected with the power source 33 through the control switch 4|. The control switch 49 is of the time delay opening type actuated by a solenoid 5| which is connected in circuit with the power source 33 through the control switch 4|. Upon the closure of the control switch 4| to set the generator I in operation, the switch 49 being closed, both the main switch 29 and the auxiliary switch 43 will close, due to the energizing of their respective operating solenoids 35 and 41. Operating potential, when both switches 29 and 43 are closed, will be' delivered through the auto transformer 3| and the switch 29 to the transformer. Upon the opening of the switch 29 at the conclusion of the exposure interval determined by the timing switch 39, the auxiliary switch 43 may remain closed to thereby continue the delivery of potential from the auto transformer through the limiting means 45 to the generator, thereby maintaining in the generator a potential insufficient to produce effective X- rays, but sufficient to prevent the anode from cooling at a destructively rapid rate.

It has been found necessary to continue the reduced potential only for a period of one-half to one second and for this reason the time delay opening switch 49 may conveniently be adjusted to open after an interval of between one-half and one second, say three-quarters of a second, after the initial closure of the switch 4| so that the arrangement serves to maintain the delayed anode heating potential only when the timing switch 39 is set for fractional second intervals less than the interval for which the delay opening switch is set, it being unnecessary to provide for the maintenance of the potential where the timing switch 39 is set for relatively long exposure intervals of the order of one-half second or more. It is, in fact, undesirable to maintain the potential longer than is necessary to regulate anode merely adds to the total heat. and is therefore undesirable. Where the timing switch 39 is ad-.

justed-to an exposure interval of longer duration than the interval for which the delay opening relay switch 49 is set, said switch, of course, will act to open the auxiliary switch means 43 in advance of the opening of the switch means 29.

The foregoing generator control arrangement has proved to be exceedingly effective in prolonging the service life of X-ray generators and to successfully prevent failure of generators through the development of anode cracks. The arrangement is simple and inexpensive, requires little,- if any, attention beyond that needed to main-;

tain the essential control equipment, yet the improvement results in a material saving in the X-ray generator, which, in the last analysis, is the most expensive unit employed in X-ray generating apparatus.

It is thought that the invention and its numerous attendant advantages will be fully understood from the foregoing description, and it is obvious that numerous changes may be made in the form, construction and arrangement of the several parts without departing from the spirit or scope of the invention, or sacrificing any ofits attendant advantages, the form herein disclosed being a preferred embodiment for the purpose of illustrating the invention.

The invention is hereby claimed as follows:

1. The method of operating an X-ray generator which consists in applying operating potential between the anode and cathode of the generator for the production of useful X-rays during a limited exposure interval, and, during a period other than said interval, delivering electrical anode heating energy to said anode whereby to retard cooling of the anode, following said interval, sufiiciently to prevent cracking thereof.

2. The method of operating an X-ray generator which consists in applying operating potential between the anode and cathode of the generator for the production of useful X-rays during a limited time interval of duration insufficient to materially heat the body of the anode while appreciably heating the target surface thereof, and, after the expiration of the interval, maintaining a reduced potential between the anode and cathode for a period of time to retard the cooling of the target surface of the anode sufiiciently to avoid cracking of the target face in cooling to the temperature of the body.

3. Apparatus for the control of an X-ray generator having an anode providing a target sur-' face and a cathode for delivering electrons upon said surface, comprising means for applying an operating potential between the anode and cathode of the generator for the production of useful X-rays at said surface during a limited time interval, and means for maintaining, between the anode and cathode, after the expiration of the interval, a reduced potential to retard the cooling of the target surface of the anode within a safe cooling rate to prevent anode cracking during cooling of the target surface.

4. Apparatus for the control of an X-ray generator having an anode providing a target surface and a cathode for delivering electrons upon said surface, comprising means for applyim, an operating potential between the anode and cathode of the generator for the production of useful X-rays during a limited time interval, and means for maintaining a reduced potential between the duringa limited time interval, and means for maintaining, between the anode and cathode, for a limited period after the expiration of the time interval, a reduced potential to control the cooling of the target surface, comprising an auxlliary in parallel relationship with respect to the main circuit, said auxiliary circuit including time delay switch means for opening the auxiliary switch means after the expiration of said limited period.

6. Apparatus for the control of an X-ray generator having an anode providing a target surface and a cathode for delivering electrons on said surface, comprising a main circuit including main switch means for applying an operating potential between the anode and cathode of the generator for the production of useful X-rays, time switch means operable to close and hold said main switch means closed during a limited time interval, and means for maintaining a reduced potential between the anode and cathode during a limited period after the expiration of the time interval to control the cooling of said target surface, comprising an auxiliary circuit in parallel relationship with respect to said main circuit, said auxiliary circuit including auxiliary switch means and time delay means for opening the auxiliary switch means,

7. Apparatus for the control of an X-ray generator having an anode providing a target surface and a cathode for delivering electrons upon said surface, comprising a main circuit including main switch means for applying an operating potential between the anode and cathode of the generator for the production of useful X-rays, time switch means operable under control of a master control switch to close and hold said main switch means closed during a limited time interval, and an auxiliary circuit adapted to apply a reduced potential between the anode and cathode, said auxiliary circuit comprising auxiliary switch means closable with said main switch under control of said master control switch and openable after expiration of a time delay interval controlled by a time delay relay.

8. Apparatus for the control of an X-ray generator having an anode providing a target surface and a. cathode for delivering electrons upon said surface, comprising a main circuit including main switch means for applying an operating potential between the anode and cathode of the generator for the production of useful X-rays, time switch means operable under control of a master control switch to close and hold said main switch means closed during a limited time interval, and an auxiliary circuit operable to supply electrical anode heating energy to the anode when said anode is free of said operating potential to thus regulate transient anode temperatures and retard cooling of the anode target surface within a safe rate to avoid cracking thereof.

9. Apparatus for the control of an X-ray generator having an anode providing a target surface and a cathode for delivering electrons upon said surface, comprising means for applying an operating potential between the anode and cathode of the generator for the production of useful Xrays at said surface during a limited exposure interval, and circuit means for heating the anode during a period other than said interval to thus regulate transient anode temperatures and prevent cracking of the target surface through too rapid cooling thereof following the exposure interval.

10. Apparatus for the control of an X-ray generator having an anode providing a target surface and a cathode for delivering electrons upon said surface, comprising a main circuit including main switch means for applying an operating potential between the anode and cathode of the generator for the production of useful X-rays, time switch means operable under control of a master control switch to close and hold said main switch means closed during a limited time interval, and means for applying a reduced potential between the anode and cathode during a limited period, comprising an auxiliary circuit including auxiliary switch means and time delay relay means for controlling the auxiliary switch, said relay comprising a switch operating solenoid, a relay switch controlling said solenoid, and a relay coil controlling said relay switch, said relay coil in turn being controlled by said master switch to energize the solenoid and close the auxiliary switch means upon operation of the master switch to close the main switch, and time delay means to open said relay switcl and thereby open the auxiliary switch means at the expiration of said limited period following operation of the master switch.

ZED J. ATLEE. 

